Pickling of magnesium and its alloys



Patented Feb. 28, 1950 2,499,299 PICKLINGI F MAGNESIUM AND rrs more Herbert K. De Long, Midland, Mich., assignor to The Dow Chemical Company. M dland, Mich.I a corporation of Delaware No Drawing. Application October c, 1947,

Serial No. 779,292

3 Claims. 1

The invention relates to methods of treating the surface of articles of magnesium and the magnesium-base alloys containing at least about 85 per cent of magnesium. It more particularly concerns an improved pickling treatment for 5 rendering the surface of articles of magnesium and magnesium-base alloys free from oxldic materials.

The conventional pickling procedures for the magnesium and magnesium allo articles employ a liquid reagent which attacks and dissolves the oxidic material that is always present on the articles as conventionally fabricated. While these pickling liquids remove most, if not all, the original oxidlc coating normally present on the surl5 face of conventionally fabricated articles, they all have the disadvantage of forming on the surface a tenuous oxidic film which is either invisible to the eye but evident upon examination with the aid of the electron microscope or visible as a discoloration which often appears smutty particularly when the magnesium alloy contains aluminum. The film, when invisible, is also evidenced by the fact that the article cannot be plated by mere immersion in certain zinc containing solutions, as described and claimed in my copending application, Serial No. 778,283, filed October 6, 1947 filed concurrentl herewith. 0n the other hand when this invisible film is removed,

a bare surface is obtained which may be plated readily with metallic zinc by mere immersion in the zincing solution as described in the said application. It is the principal object of the invention to provide an improved pickling method for articles of magnesiumand its alloys whereby the surface is left substantially free from oxidic material.

' other objects and advantages will appear as the description of the invention proceeds.

I have discovered that" aqueous solutions of face of all conventionally fabricated magnesium" and this result is obtained without forming the:

oxidic film mentioned above. In carrying out the invention, the article of a magnesium or magnesium-base alloy first may be given a conventional cleaning, if desired, as"

by either a conventional pickle, such as the usual; "chrome-pickle" or a mechanical operation suclrifl as sanding, wire brushing, or the like, followed bvggp a dcgreasingstep so as toremove most of the sur facecontamination it, the surface is excessively dirty. According to the invention, the article is treated with the aqueous aluminum chloride solution in a manner to maintain the solution and the article in contact, as by immersing the article in the solution or by spraying the solution on the article.

For preparing the solution either the anhydrous form (AlCla) or the hydrous form (AlClaBHzO) may be used. Suitable concentrations are from about 5 grams to 250 grams or more of A101: per liter of solution. A preferred concentration is about grams to 165 grams per liter, while a solution containing about grams per liter is generally satisfactory for most pickling operations.

The duration of the treatment may be varied to suit the conditions of the surface of the article to be treated, the temperature of the aluminum chloride solution, and its concentration. The heavier the coating of surface contamination to be removed, the colder and more dilute the.

solution,'the longer the treatment time that is required to eflect cleaning. In the usual operation of the bath, its temperature may be between about 10 C. and 60 C., for example, although other temperatures may be used, preferably room temperature. In the foregoing temperature range and concentrations, there may be used an immersion of from about 5 seconds which suiilces for light deposits with the warmer and more concentrated baths to 5 minutes which suillces for dense deposits and the cooler more dilute baths. In general, an immersion of about 30 seconds at room temperature, in a bath containingabout 110 grams of MCI: per liter suffices for pickling the average article.

The following examples are illustrative of the practice of the method:

Example 1 A magnesium-base alloy die-casting having a nominal composition of 9 per cent aluminum, 0.2 per cent manganese, 0.6 per cent zinc, the balance being magnesium, in the as cast condition, was pickled. This consisted of an immersion for 1 minute in a 11.0 per cent aqueous solution of aluminum chloride (A1013) at about 25 C. The

, pickled casting was then removed and rinsed. Be-

fore the treatment, the surface of the casting bore a covering of the product of the usual atmospheric oxidation resulting from atmospheric exposure while the hot casting left the die-casting machine and during the several days afterward prior to the pickling. After he i llckling treatnient, the surface-was'free from the product of atmospheric oxidation and examination under the electron microscope showed tlrst the surface of the metal was substantially free of any coating.

Example 2 A specimen of a magnesium-base alloy rolled sheet'having a nominal composition of 3 per cent aluminum, 0.3 per cent manganese, l per cent zinc, the balance being magnesium and an oxidized surface resulting from atmospheric exventionally pickled with a per cent solution of nitric acid in water and then immersed in the zincing solution did not become coated with zinc. The above-mentioned zincing solution and zincing method is set forth in the aforementioned copending application.

Among the advantages of the invention are that a simple solvent treatment suffices to remove surface contamination without leaving an oxidic film. The method is rapid and is elective on surfaces of simple as well as complex shape.

I claim: I l. The method of pickling an article of magnesium or magnesium-base alloys containing at least 85 per cent of magnesiumwhich comprises dissolving the surface contaminants by treating the article with an aqueous solution of aluminum chloride.

2. The method of pickling an article of magnesium or magnesium-base alloys containing at least 85 per cent of magnesium which comprises dissolving the surface contaminants by immersing the article in an aqueous solution of aluminum chloride having a concentration of about 5 to 250 grams of A101: perliter of solution.

3. The method of pickling an article of magnesium or magnesium-base alloys containing at least per cent of magnesium which comprises immersing the article in an aqueous solution of aluminum chloride having a concentration of about to grams of MCI: per liter of solution and a temperature between about 10 and 60 C. for about 5 seconds to 30 minutes, rinsing, and drying. I

HERBERT K. DE LONG.

No references cited. 

3. THE METHOD OF PICKLING AN ARTICLE OF MAGNESIUM OR MAGNESIUM-BASE ALLOYS CONTAINING AT LEAST 85 PER CENT OF MAGNESIUM WHICH COMPRISES IMMERSING THE ARTICLE IN AN AQUEOUS SOLUTION OF ALUMINUM CHLORIDE HAVING A CONCENTRATION OF ABOUT 80 TO 165 GRAMS OF ALCL3 PER LITER OF SOLUTION AND A TEMPERATURE BETWEEN ABOUT 10* AND 60*C. FOR ABOUT 5 SECONDS TO 30 MINUTES, RINSING, AND DRYING. 